Traffic collision-avoidance systems (TCAS) use two antennas, one on the top and one on the bottom of an aircraft, each of which is used to estimate the relative bearing between one's own aircraft and an intruder. Each traditional TCAS antenna has four elements, which are placed orthogonally on the same plane, as shown in FIG. 1. A TCAS system interrogates the transponder on the other aircraft. The transponders of other aircraft respond with a reply that may contain altitude or other information. The TCAS uses the reply signal and its multi-element directional antenna to estimate the relative bearing of the other aircraft. In some systems, the phase difference of the reply signal received at elements E1 and E3 is proportional to the sine function value of the intruder's bearing angle, and the phase difference between elements E2 and E4 is proportional to the cosine function value of the intruder's bearing angle (where the E1/E3 pair and the E2/E4 pair are orthogonal). The system can estimate the bearing from the signals received on one multi-element antenna. Although this is an adequate setup for determining bearing, it is redundant, costly, and comes with a weight penalty because of the two antennas and eight antenna elements that are needed.
A bearing calculated from a single reply, squitter, or their combination, might be corrupted at the point of reception by interference (multipath, reflection from airframe or nearby obstacles). Even under stationary settings, such corrupted raw bearings vary to the extent that, on display (if not heavily filtered), it looks like a moving target. However, such movements do not look natural and may decrease confidence in the system.